This is a very detailed article about using SYSTEM_THREAD(ENABLED); (or
not), and how it affects publishing and other networking, and how to
properly avoid your code blocking on certain operations.
https://community.particle.io/t/particle-publish-and-blocking/42022
You may not even realize that the Particle Device OS is built atop FreeRTOS, a real-time operating system for embedded devices, and that FreeRTOS supports Threads, which allow you to run multiple tasks simultaneously or asynchronously. However, threads are very complex and there are lots of pitfalls. This article gives a pretty deep dive into using threads, and why you probably should avoid it, unless you really know what you're doing.
https://github.com/rickkas7/particle-threads